Topic 5 Energy In Society Flashcards
What is tonnes of oil equivalent (TOE)?
This is the total energy released when a tonne (1000 kg) of crude oil is burned, which is taken to be 41.9 GJ.
Since the energy use of society is so large, often the mega tonne of oil equivalent is used (Mtoe).
What is efficiency? (equation)
In a physical process, efficiency is defined as the amount of work output divided by the amount of energy supplied. It is therefore a dimensionless number often specified as a percentage.
Efficiency = (work output) / (energy supplied)
= W / Esup
What is specific energy?
This is the total energy released from 1 kg of a fuel or power source.
Steps for calculating the energy change in a reaction.
1 - write down all the bonds from the starting material that must be broken.
2- calculate the energy input required to break all the bonds identified in step one.
3- identify all the bonds that are formed on the right hand side.
4- calculate the energy released when all the right hand side bonds are formed.
5 - add the energy calculated from step 2 to the energy calculated in step 4. The sum is the energy absorbed or released by the reaction.
What is an endothermic reaction?
This is any reaction where the change in energy is positive (the energy of the system increases).
Breaking bonds is endothermic.
What is an exothermic reaction?
This is where the energy change is negative (the energy of the system decreases).
Making bonds is exothermic.
What is ** bond enthalpy**?
This is the energy required to break a chemical bond or the amount of energy released when forming a chemical bond.
This is typically measured in kilojoules per mole.
What in a mole?
This is an amount of a substance that contains a number of atoms or molecules equal to Avogadro’s constant (6.022 x 10^23).
What is molar mass?
This is the mass of one mole of a substance. It is defined that one mole of carbon 12 has a mass of 12.0 g. In general one mole of a substance has a molar mass in grams that is equal to its relative atomic / molecular mass.
What is the atomic mass unit?
The atomic mass unit (u) is one twelfth of the mass of a single atom of carbon 12, which is equal to 1.66 x 10^-27 kg.
The mass of any atom or molecule is equal to u times its relative atomic or molecular mass.
What is nuclear fission?
The process whereby an unstable atomic nucleus splits into two roughly equal halves. There is a decrease in the total mass of the nuclei in a fission reaction, so this process results in the release of large amounts of energy according to Einsteins E = mc^2.
What is EROI? (equation)
This stands for the energy return on investment and quantifies how much energy can be extracted from a fuel that costs one unit of energy to obtain.
EROI = (energy extracted) / (energy expended)
What is hydroelectric power?
This is produced by allowing water to pass through a turbine by falling through a height.
What is wind power?
This is generated when winds are used to directly turn turbines that are used to generate electricity.
What is solar power?
This makes use of photovoltaic cells, which produce electricity from light.
What is heat?
Heat is the transfer of energy that occurs as a result of a temperature difference.
Rate of energy transfer (equation).
The rate of energy transfer is proportional to the temperature difference across the boundary.
ΔT = Tin - Tout
Specified in kelvin.
Calculate the rate of energy transfer (power equation)
P = E / t
Where the rate if energy transfer is power, P, amount of energy transfer is E and t is the time in which the energy is transferred.
Rate of energy transfer relationships equations (heat conduction equation 2.1)
P = power, A = surface area and l = boundary thickness, T = temperature (K)
P α Α
P α ΔΤ
P α 1/l
P α AΔΤ/l
P = -(kA (ΔT)/l) (2.1)
Where k is a constant of proportionality which changes depending on the material.
Calculating the Chang in temperature across multiple layers.
ΔT = -(P/A)(la/ka + lb/kb)
Where P is power, A is surface area and lx and kx are the thickness and thermal conductivity values for each layer.
For more layers you can just add more terms for lx and kx inside the second bracket.
Radiation power equation.
P rad = AεσΤ^4
Where ε is the emissivity and σ is the stephan Boltzmann constant. A is area and T is temp in kelvin.
To calculate the net power being emitted its the difference between the power radiated and the radiation received by the radiator.
What is rolling resistance?
A type of energy dissipation experienced by wheels rolling over a surface.
Occurs due to the deformation of the wheel as it is squashed and unsquashed by the weight of the car.
Equation for the energy dissipated due to rolling
Work (tyres) = F(tyres) x d
Work (tyres) = Mgd
Energy (rr) = C(rr) W(tyres)
= C(rr)Mgd
Where Crr is the coefficient of rolling resistance.
Equation for the power dissipated through a type.
P(rr) = E(rr) / t
P(rr) = (C(rr) Mgd) / t
As v = d/t
P(rr) = C(rr) Mgv
Air resistance formula
P(air) = (1/2)ρC(D)Av^3
Where ρ is air density, C(D) is the drag coefficient which is unitless, A is the frontal cross sectional area and v is the velocity
Comparing air resistance at 2 speeds formula
P(air v2) = (v2/v1)^2 P(air v1)
Calculating friction (basic model)
As a general rule of thumb, approximately 15% of the total power is lost to the transmission due to fiction. This is the some as saying 85% of the power from the engine is delivered to the wheels.
Proportionality between forces
P(air) α air density
P(air) α speed^3
P(air) α cross sectional area
P(air) α
P(rr) α weight
P(rr) α speed
What happens when a car brakes?
The kinetic energy is dissipated mainly as fiction in the braking system.
